U.S. patent number 5,985,434 [Application Number 08/977,918] was granted by the patent office on 1999-11-16 for absorbent foam.
This patent grant is currently assigned to Kimberly-Clark Worldwide, Inc.. Invention is credited to Jian Qin, Palani Raj Ramaswami Wallajapet, Gary D. Williams.
United States Patent |
5,985,434 |
Qin , et al. |
November 16, 1999 |
Absorbent foam
Abstract
Disclosed is an absorbent foam that exhibits desirable softness
and flexibility properties yet is highly absorbent. In one
embodiment, the absorbent foam comprises a water-swellable,
water-insoluble polymer wherein the absorbent foam exhibits a Free
Swell value of at least about 10 grams of liquid per gram of
absorbent foam and a Softness value that is less than about 30
grams of force per gram per square meter of absorbent foam. In a
second embodiment, the absorbent foam has an average cell size of
the cells in the absorbent foam between about 10 microns to about
100 microns and an average wall thickness of the cells in the
absorbent foam between about 0.1 micron to about 30 microns. Such
an absorbent foam may be used in a disposable absorbent product
intended for the absorption of fluids such as body fluids.
Inventors: |
Qin; Jian (Appleton, WI),
Wallajapet; Palani Raj Ramaswami (Wauwatosa, WI), Williams;
Gary D. (Neenah, WI) |
Assignee: |
Kimberly-Clark Worldwide, Inc.
(Neenah, WI)
|
Family
ID: |
25525645 |
Appl.
No.: |
08/977,918 |
Filed: |
November 25, 1997 |
Current U.S.
Class: |
428/315.5;
428/315.9; 521/66; 521/905; 604/369; 604/378 |
Current CPC
Class: |
A61L
15/22 (20130101); A61L 15/425 (20130101); C08J
9/28 (20130101); Y10T 428/249978 (20150401); C08J
2300/00 (20130101); Y10S 521/905 (20130101); Y10T
428/24998 (20150401); C08J 2201/052 (20130101) |
Current International
Class: |
A61L
15/22 (20060101); A61L 15/42 (20060101); A61L
15/16 (20060101); C08J 9/00 (20060101); C08J
9/28 (20060101); A61F 013/15 (); B32B 003/26 () |
Field of
Search: |
;428/315.5,315.9
;521/66,905 ;604/369,358,378 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
American Society for Testing Materials (ASTM) Designation: D
4032-82, "Standard Test Method for Stiffness of Fabric by the
Circular Bend Procedure," pp. 702-706, published Aug. 1982. .
Good, Robert J. and Robert J. Stromberg, Editors, Surface and
Colloid Science--Experimental Methods, vol. II, Plenum Press, Dec.
1979, pp. 31-91 ..
|
Primary Examiner: Copenheaver; Blaine
Attorney, Agent or Firm: Jones & Askew, LLP
Claims
What is claimed is:
1. An absorbent foam comprising a water-swellable, water-insoluble
polymer wherein the water-swellable, water-insoluble polymer is
present in the absorbent foam in a weight amount between about 50
weight percent to 100 weight percent, based on the total weight of
the absorbent foam, and wherein the absorbent foam exhibits a Free
Swell value of at least about 10 grams of liquid per gram of
absorbent foam and a Softness value that is less than about 30
grams of force per gram per square meter of the absorbent foam.
2. The absorbent foam of claim 1 wherein the polymer is selected
from the group consisting of polyacrylamides, polyvinyl alcohols,
ethylene maleic anhydride copolymer, polyvinylethers, polyacrylic
acids, polyvinylpyrrolidones, polyvinylmorpholines, polyamines,
polyethyleneimines, polyquaternary ammoniums, carboxymethyl
celluloses, carboxymethyl starchs, hydroxypropyl celluloses,
algins, alginates, carrageenans, acrylic grafted starchs, acrylic
grafted celluloses, chitin, chitosan, polyaspartic acid,
polyglutamic acid, polyasparagins, polyglutamines, polylysines,
polyarginines, and the salts, copolymers, and mixtures of any of
the foregoing polymers.
3. The absorbent foam of claim 2 wherein the polymer is selected
from the group consisting of polyacrylic acids, carboxymethyl
celluloses, chitin, chitosan, and the salts, copolymers, and
mixtures of any of the foregoing polymers.
4. The absorbent foam of claim 3 wherein the water-swellable,
water-insoluble polymer is selected from the group consisting of
polyacrylic acids and its salts.
5. The absorbent foam of claim 1 wherein the water-swellable,
water-insoluble polymer is present in the absorbent foam in a
weight amount between about 60 weight percent to 100 weight
percent.
6. The absorbent foam of claim 1 wherein the absorbent foam further
comprises a crosslinking agent.
7. The absorbent foam of claim 6 wherein the crosslinking agent is
selected from the group consisting of an organic compound having at
least two functional groups or functionalities capable of reacting
with the polymer and a metal ion with two or more positive
charges.
8. The absorbent foam of claim 6 wherein the crosslinking agent is
present in the absorbent foam in a weight amount between about 0.01
weight percent to about 20 weight percent, based on the total
weight of the absorbent foam.
9. The absorbent foam of claim 1 wherein the absorbent foam
exhibits a Free Swell value of at least about 15 grams of liquid
per gram of absorbent foam.
10. The absorbent foam of claim 1 wherein the absorbent foam
exhibits a Softness value that is less than about 25 grams of force
per gram per square meter of the absorbent foam.
11. The absorbent foam of claim 1 wherein the absorbent foam
exhibits an Absorbency Under Load value of at least about 10 grams
of liquid per gram of absorbent foam.
12. The absorbent foam of claim 1 wherein the absorbent foam
comprises cells and wherein the average cell size of the cells is
between about 10 microns to about 100 microns.
13. The absorbent foam of claim 1 wherein the absorbent foam
comprises cells comprising walls having a thickness and wherein the
average wall thickness of the cells is between about 0.1 micron to
about 30 microns.
14. The absorbent foam of claim 1 wherein the water-swellable,
water-insoluble polymer is selected from the group consisting of
polyacrylic acids, carboxymethyl celluloses, chitin, chitosan, and
the salts, copolymers, and mixtures of any of the foregoing
polymers, wherein the absorbent foam comprises cells comprising
walls having a thickness wherein the average cell size of the cells
is between about 10 microns to about 100 microns, and wherein the
average wall thickness of the cells is between about 0.1 micron to
about 30 microns.
15. An absorbent foam comprising a water-swellable, water-insoluble
polymer, wherein the water-swellable, water-insoluble polymer is
present in the absorbent foam in a weight amount between about 50
weight percent to about 100 weight percent, wherein the absorbent
foam comprises cells comprising walls having a thickness wherein
the average cell size of the cells is between about 10 microns to
about 100 microns, and wherein the average wall thickness of the
cells is between about 0.1 micron to about 30 microns, further
wherein the absorbent foam has a softness value that is less than
about 30 grams of force per gram per square meter of the absorbent
foam.
16. The absorbent foam of claim 15 wherein the polymer is selected
from the group consisting of polyacrylamides, polyvinyl alcohols,
ethylene maleic anhydride copolymer, polyvinylethers, polyacrylic
acids, polyvinylpyrrolidones, polyvinylmorpholines, polyamines,
polyethyleneimines, polyquaternary ammoniums, carboxymethyl
celluloses, carboxymethyl starchs, hydroxypropyl celluloses,
algins, alginates, carrageenans, acrylic grafted starchs, acrylic
grafted celluloses, chitin, chitosan, polyaspartic acid,
polyglutamic acid, polyasparagins, polyglutamines, polylysines,
polyarginines, and the salts, copolymers, and mixtures of any of
the foregoing polymers.
17. The absorbent foam of claim 16 wherein the water-swellable,
water-insoluble polymer is selected from the group consisting of
polyacrylic acids, carboxymethyl celluloses, chitin, chitosan, and
the salts, copolymers, and mixtures of any of the foregoing
polymers.
18. The absorbent foam of claim 17 wherein the water-swellable,
water-insoluble polymer is selected from the group consisting of
polyacrylic acids and its salts.
19. The absorbent foam of claim 15 wherein the water-swellable,
water-insoluble polymer is present in the absorbent foam in a
weight amount between about 60 weight percent to 100 weight
percent.
20. The absorbent foam of claim 15 wherein the absorbent foam
further comprises a crosslinking agent.
21. The absorbent foam of claim 20 wherein crosslinking agent is
selected from the group consisting of an organic compound having at
least two functional groups or functionalities capable of reacting
with the polymer and a metal ion with two or more positive
charges.
22. The absorbent foam of claim 21 wherein the crosslinking agent
is present in the absorbent foam in a weight amount between about
0.01 weight percent to about 20 weight percent, based on the total
weight of the absorbent foam.
23. The absorbent foam of claim 15 wherein the absorbent foam
exhibits a Free Swell value of at least about 10 grams of liquid
per gram of absorbent foam.
24. A disposable absorbent product comprising a liquid-permeable
topsheet, a backsheet attached to the topsheet, and an absorbent
core positioned between the liquid-permeable topsheet and the
backsheet, wherein the absorbent core comprises an absorbent foam,
wherein the absorbent foam comprises a water-swellable,
water-insoluble polymer wherein the water-swellable,
water-insoluble polymer is present in the absorbent foam in a
weight amount between about 50 weight percent to 100 weight
percent, based on the total weight of the absorbent foam, and
wherein the absorbent foam exhibits a Free Swell value of at least
about 10 grams of liquid per gram of absorbent foam and a Softness
value that is less than about 30 grams of force per gram per square
meter of the absorbent foam.
25. A disposable absorbent product comprising a liquid-permeable
topsheet, a backsheet attached to the topsheet, and an absorbent
core positioned between the liquid-permeable topsheet and the
backsheet, wherein the absorbent core comprises an absorbent foam,
wherein the absorbent foam comprises a water-swellable,
water-insoluble polymer wherein the water-swellable,
water-insoluble polymer is present in the absorbent foam in a
weight amount between about 50 weight percent to 100 weight
percent, wherein the absorbent foam comprises cells comprising
walls having a thickness wherein the average cell size of the cells
is between about 10 microns to about 100 microns, and wherein the
average wall thickness of the cells is between about 0.1 micron to
about 30 microns, further wherein the absorbent foam has a softness
value that is less than about 30 grams of force per gram per square
meter of the absorbent foam.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an absorbent foam that exhibits
desirable softness and flexibility properties yet is highly
absorbent. Such an absorbent foam may be used in a disposable
absorbent product intended for the absorption of fluids such as
body fluids.
2. Description of the Related Art
Disposable absorbent products currently find widespread use in many
applications. For example, in the infant and child care areas,
diapers and training pants have generally replaced reusable cloth
absorbent articles. Other typical disposable absorbent products
include feminine care products such as sanitary napkins or tampons,
adult incontinence products, and health care products such as
surgical drapes or wound dressings. A typical disposable absorbent
product generally comprises a composite structure including a
topsheet, a backsheet, and an absorbent structure between the
topsheet and backsheet. These products usually include some type of
fastening system for fitting the product onto the wearer.
The use of water-swellable, generally water-insoluble absorbent
materials, commonly known as superabsorbents, in disposable
absorbent personal care products is known. Such absorbent materials
are generally employed in absorbent products in order to increase
the absorbent capacity of such products while reducing their
overall bulk. Such absorbent materials are generally present in
absorbent products in the form of small particles in a fibrous
matrix, such as a matrix of wood pulp fluff. A matrix of wood pulp
fluff generally has an absorbent capacity of about 6 grams of
liquid per gram of fluff. The superabsorbent materials generally
have an absorbent capacity of at least about 10, preferably of
about 20, and often of up to 100 times their weight in water.
Clearly, incorporation of such absorbent materials in disposable
absorbent products can reduce the overall bulk while increasing the
absorbent capacity of such products.
As an alternative to using a fibrous matrix containing
superabsorbent materials, absorbent foam composites are also known.
One form of an absorbent foam composite is wherein a foam material,
such as polyurethane, is prepared to include a particulate
superabsorbent material within the structure of the polyurethane
foam. Alternatively, a particulate superabsorbent material is
located between at least two layers of a polyurethane foam material
to form a layered composite structure. While such foam structures
may be useful absorbent materials in specific applications, they
have not been shown to be optimal for use in disposable absorbent
products because their absorptive properties tend to be limited. In
particular, the foam material in such structures, such as
polyurethane, generally does not have a sufficient absorptive
ability to retain liquids. Therefore, although the particulate
superabsorbent material in the foam structure may be able to retain
a liquid, the overall capacity of the foam structure to absorb and
retain a liquid is limited. Furthermore, the overall absorptive
properties of the foam structure tend to be limited due to the
relatively low surface area to mass ratio of the particulate
superabsorbent material portion relative to the foam portion of the
structure.
Absorbent foams are also known that are prepared comprising
essentially all superabsorbent material. Typically, a blowing agent
is used to form a foamed, water-swellable, polymeric liquid
absorbent material. However, certain absorbent foams prepared using
specific blowing agents have been found to have limited use for
liquid absorption or liquid distribution. This is typically due to
physical characteristics of the foam structure, which may include
discontinuous channels, a too large average cell size, unacceptably
wide cell size distribution, and/or capillary diameters that vary
widely and randomly, that tend to result in undesirable absorptive
rates and capacities and undesirable liquid distribution
properties. In addition, known absorbent foams that are prepared
comprising essentially all superabsorbent material have typically
been found to have undesirable non-absorptive physical
characteristics such as a lack of softness or being too brittle.
Furthermore, many of the known foams are hydrophobic in nature and
need treatment with a wetting agent or other suitable treatment
steps to obtain a hydrophilic nature. Such undesirable
non-absorptive physical characteristics of an absorbent foam tends
to limit the usefulness of the absorbent foam in disposable
absorbent products since such disposable absorbent products
generally need to be sufficiently flexible to withstand the rigors
of use by a consumer and also be sufficiently soft to be acceptably
comfortable during use.
Thus, there is a continuing need for improvement of absorbent
foams. In particular, there is a need for an absorbent foam which
exhibits a relatively high absorptive liquid capacity yet which
exhibits desirable softness and flexibility properties.
It is therefore an object of the present invention to provide an
absorbent foam which exhibits a relatively high absorptive liquid
capacity yet which exhibits desirable physical characteristics such
as softness and flexibility properties.
It is also an object of the present invention to provide a
disposable absorbent product which includes an absorbent foam that
exhibits a relatively high absorptive liquid capacity yet which
exhibits desirable physical characteristics such as softness and
flexibility properties.
SUMMARY OF THE INVENTION
The present invention concerns an absorbent foam which exhibits a
relatively high absorptive liquid capacity yet which exhibits
desirable physical characteristics such as softness and flexibility
properties.
One aspect of the present invention concerns an absorbent foam that
comprises a water-swellable, water-insoluble polymer wherein the
absorbent foam exhibits a Free Swell value of at least about 10
grams of liquid per gram of absorbent foam and a Softness value
that is less than about 30 grams of force per gram per square meter
of absorbent foam.
In another aspect, the present invention concerns a absorbent foam
that comprises a water-swellable, water-insoluble polymer wherein
the absorbent foam has an average cell size of the cells in the
absorbent foam between about 10 microns to about 100 microns and an
average wall thickness of the cells in the absorbent foam between
about 0.1 micron to about 30 microns.
In another aspect, the present invention concerns a disposable
absorbent product comprising the absorbent foam disclosed
herein.
One embodiment of such a disposable absorbent product comprises a
liquid-permeable topsheet, a backsheet attached to the
liquid-permeable topsheet, and an absorbent foam of the present
invention located between the liquid-permeable topsheet and the
backsheet.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an illustration of the equipment employed in determining
the Free Swell and Absorbency Under Load values of an absorbent
foam or material.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention is directed to an absorbent foam which
exhibits a relatively high absorptive liquid capacity yet which
exhibits desirable softness and flexibility properties. The
absorbent foam comprises a water-swellable, water-insoluble
polymer. As used in the present invention, the water-swellable,
water-insoluble polymer to a large extent needs to provide the
absorbent foam with its liquid-absorbing capacity. As such, the
water-swellable, water-insoluble polymer needs to be effective to
provide a desired amount of liquid-absorbing capacity to the
absorbent foam.
As used herein, the term "foam" is generally intended to represent
a porous polymeric matrix, which is an aggregate of hollow cells,
the boundaries or walls of which cells comprise solid polymeric
material. The cells may be interconnected to form channels or
capillaries within the foam structure wherein such channels or
capillaries facilitate liquid distribution within the foam.
As used herein, the term "water-swellable, water-insoluble" is
meant to refer to a material that, when exposed to an excess of
water, swells to its equilibrium volume but does not dissolve into
the water. As such, a water-swellable, water-insoluble material
generally retains its original identity or physical structure, but
in a highly expanded state, during the absorption of the water and,
thus, must have sufficient physical integrity to resist flow and
fusion with neighboring materials.
As used herein, a material will be considered to be "water soluble"
when it substantially dissolves in excess water to form a solution,
thereby losing its initial form and becoming essentially
molecularly dispersed throughout the water solution. As a general
rule, a water-soluble material will be free from a substantial
degree of crosslinking, as crosslinking tends to render a material
water insoluble.
Polymers which are suitable for use in the present invention are
generally any polymer which is initially soluble in a solvent such
that the soluble polymer may be formed into a solution by mixing
with a liquid solvent, such as water, and then whereby the polymer
is treated to cause the polymer to become water-swellable and
water-insoluble so that an absorbent foam comprising such
water-swellable, water-insoluble polymer exhibits desired
absorbency and physical characteristics.
Polymers which are suitable for use in the present invention
include a wide variety of anionic, cationic, and nonionic
materials. Suitable polymers include polyacrylamides, polyvinyl
alcohols, ethylene maleic anhydride copolymer, polyvinylethers,
polyacrylic acids, polyvinylpyrrolidones, polyvinylmorpholines,
polyamines, polyethyleneimines, polyquaternary ammoniums, natural
based polysaccharide polymers such as carboxymethyl celluloses,
carboxymethyl starchs, hydroxypropyl celluloses, algins, alginates,
carrageenans, acrylic grafted starchs, acrylic grafted celluloses,
chitin, and chitosan, and synthetic polypeptides such as
polyaspartic acid, polyglutamic acid, polyasparagins,
polyglutamines, polylysines, and polyarginines, as well as the
salts, copolymers, and mixtures of any of the foregoing
polymers.
In one embodiment of the present invention, it is desired that the
polymer used be a glassy polymer. As used herein, the term "glassy"
polymer is meant to refer to a polymer having a glass transition
temperature (Tg) above about 23.degree. C. (about room temperature)
at a relative humidity of about 30 percent or less. Examples of
glassy polymers include, but are not limited to, sodium
polyacrylate, polyacrylic acid, sodium carboxymethyl cellulose, and
chitosan salt polymers. Examples of non-glassy polymers include,
but are not limited to, polyethylene oxide, polyvinyl acetate, and
polyvinyl ether polymers.
One property of the water-swellable, water-insoluble polymer which
is relevant to its effectiveness in providing a desired amount of
liquid-absorbing capacity to an absorbent foam is its molecular
weight. In general, a water-swellable, water-insoluble polymer with
a higher molecular weight will exhibit a higher liquid-absorbing
capacity as compared to a water-swellable, water-insoluble polymer
with a lower molecular weight.
The water-swellable, water-insoluble polymer useful in the
absorbent foam of the present invention may generally have a wide
range of molecular weights. A water-swellable, water-insoluble
polymer having a relatively high molecular weight is often
beneficial for use in the present invention. Nonetheless, a wide
range of molecular weights is generally suitable for use in the
present invention. Water-swellable, water-insoluble polymers
suitable for use in the present invention will beneficially have a
weight average molecular weight greater than about 10,000, more
beneficially greater than about 100,000, even more beneficially
greater than about 200,000, suitably greater than about 500,000,
more suitably greater than about 1,000,000, and up to about
20,000,000. Methods for determining the molecular weight of a
polymer are well-known in the art.
It is generally desired that the polymer be present in the
absorbent foam in an amount effective to result in the absorbent
foam exhibiting desired properties. The polymer will be present in
the absorbent foam in a weight amount that is between about 50
weight percent to 100 weight percent, beneficially between about 60
weight percent to about 100 weight percent, more beneficially
between about 70 weight percent to about 100 weight percent,
suitably between about 80 weight percent to about 100 weight
percent, more suitably between about 90 weight percent to about 100
weight percent, and even more suitably between about 95 weight
percent to about 100 weight percent, wherein all weight percents
are based on the total weight amount of the polymer, any
crosslinking agents, and any other optional components present in
the absorbent foam. In one embodiment of the present invention, it
is desired that the absorbent foam consist essentially of the
polymer and, optionally, any crosslinking agent used to crosslink
the polymer. As will be appreciated by one skilled in the art, such
an absorbent foam may also comprise an insubstantial amount of
solvent retained from the preparation process and/or an
insubstantial amount of water vapor absorbed from the air. In
general, the presence of any materials in the absorbent foam that
are not the water-swellable, water-insoluble polymer will tend to
reduce the overall liquid absorbency capacity of the absorbent
foam.
The water-swellable, water-insoluble polymer useful in the
absorbent foam will generally be crosslinked. The amount of
crosslinking should generally be above a minimum amount sufficient
to make the polymer water-insoluble but also below some maximum
amount so as to allow the polymer to be sufficiently water
swellable so that the water-swellable, water-insoluble polymer
absorbs a desired amount of liquid absorption.
Crosslinking of the polymer may generally occur either while the
polymer is in solution or after the solvent has been removed from a
solution used to prepare the absorbent foam. Such crosslinking of
the polymer may generally be achieved by either of two different
types of crosslinking agents. Such crosslinking agents will
generally be soluble in the solvent being used, such as water.
One type of crosslinking agent is a latent crosslinking agent.
Suitable latent crosslinking agents are generally either internal
latent crosslinking agents or external latent crosslinking agents.
An internal latent crosslinking agent is generally copolymerizable
to the monomer or monomers used to prepare the polymer and, thus,
generally comprise at least one vinyl group and one functional
group or functionality that is capable of reacting with the side
groups on the base polymer, such as a carboxyl group (--COO.sup.-)
on a sodium polyacrylate polymer or a carboxylic acid group
(--COOH) on a polyacrylic acid polymer. Examples of suitable
copolymerizable crosslinking agents include ethylenically
unsaturated monomers, such as ethylene glycol vinyl ether and amino
propyl vinyl ether.
An external latent crosslinking agent generally crosslinks the
polymer itself after, for example, a polymer has been formed from
specific monomer or monomers used to prepare the polymer and/or a
polymer has been mixed with a solvent to form a solution. Latent
crosslinking agents generally do not take part in the overall
polymerization process but, instead, are reactive to the polymer at
a later point in time when a proper crosslinking condition is
provided. Suitable crosslinking conditions include using heat
treatment, such as a temperature above about 60.degree. C.,
exposure to ultraviolet light, exposure to microwaves, steam or
high humidity treatment, high pressure treatment, or treatment with
an organic solvent.
Suitable external latent crosslinking agents are any organic
compound having at least two functional groups or functionalities
capable of reacting with the carboxyl, carboxylic acid, amino, or
hydroxyl groups of a polymer. It is desired that such an organic
crosslinking agent be selected from the group consisting of
diamines, polyamines, diols, and polyols and mixtures thereof;
particularly from the group consisting of primary diols, primary
polyols, primary diamines and primary polyamines and mixtures
thereof. Of the diols and polyols, those possessing longer, such as
4 or greater, carbon chain lengths are generally beneficial.
Specifically, the crosslinking agent may be selected from the group
consisting of chitosan glutamate, type A gelatin,
diethylenetriamine, ethylene glycol, butylene glycol, polyvinyl
alcohol, hyaluronic acid, polyethylene imine and their derivatives
and mixtures thereof. Other suitable organic crosslinking agents
include monochloroacetic acid, sodium chloroacetate, citric acid,
butane tetracarboxylic acid, and amino acids such as aspartic acid,
and mixtures thereof. Another suitable latent crosslinking agent
comprises a metal ion with more than two positive charges, such as
Al.sup.3+, Fe.sup.3+, Ce.sup.3+, Ce.sup.4+, Ti.sup.4+, Zr.sup.4+,
and Cr.sup.3+. Suitable metal ion crosslinking agents include those
of the transition elements which generally have vacant d-orbitals.
Suitable metal ion crosslinking agents include AlCl.sub.3,
FeCl.sub.3, Ce.sub.2 (SO.sub.4).sub.3, Zr(NH.sub.4).sub.4
(CO.sub.3).sub.4 and Ce(NH.sub.4).sub.4 (SO.sub.4).sub.4.2H.sub.2
O, other well known metal ion compounds and mixtures thereof. Such
metal ion crosslinking agents, when used with a particular polymer,
are believed to form ionic bonds with the carboxyl, carboxylic,
amino, or hydroxyl groups on the polymer. Metal ions with only two
positive charges, such as Zn.sup.2+, Ca.sup.2+, or Mg.sup.2+, are
also suitable as crosslinking agents for certain polymers.
When the polymer is a cationic polymer, a suitable crosslinking
agent is a polyanionic material such as sodium polyacrylate,
carboxymethyl cellulose, or polyphosphate.
A second type of crosslinking mechanism that certain polymers are
able to undergo involves a macromolecular rearrangement of the
chains of the polymer during the solidification process of the
polymer such that the polymer forms a higher ordered structure with
a high degree of crystallinity which is generally water insoluble.
Polymers suitable to such a crosslinking approach include, but are
not limited to, polyvinyl alcohol, chitosan, and carboxymethyl
cellulose with a relatively low degree of carboxymethylation.
Additional strong bonding of the polymer could be established
between the polymer chains during the solidification process which
could result in a generally water insoluble material. An example of
this behavior is the strong hydrogen bonding in polyvinyl alcohol
forming an insoluble material.
Suitable crosslinking agents for a polymer solution gel process are
also generally of two different types: either internal
polymerizable or external crosslinking agent. The first type of
crosslinking agent is a polymerizable but instant crosslinking
agent. Suitable polymerizable crosslinking agents are generally
reactive to the monomer or monomers used to prepare the polymer
and, thus, generally comprise at least two functional groups or
functionalities that are capable of reacting with the monomers.
Examples of suitable polymerizable crosslinking agents include
ethylenically unsaturated monomers, such as N,N'-methylene
bis-acrylamide for free radical polymerization, and polyamines or
polyols for condensation polymerization. The second type of
crosslinking agent is a reactive compound having at least two
functional groups or functionalities capable of reacting with the
carboxyl, carboxylic acid, amino, or hydroxyl groups of a polymer
in the solution stage wherein such crosslinking is not latent, in
that no additional conditions are needed to initialize the
crosslinking reaction. Suitable crosslinking agents may be selected
from the group consisting of aldehydes, such as glutaraldehyde, or
glycidyl ethers, such as polyethylene gylcol diglycidyl ether.
Another approach to form a crosslinked polymer network in either a
polymer solution or on a recovered polymer is the use of a high
energy treatment such as electron beam radiation or microwave
radiation to form free radicals in the polymer which are then used
to generate crosslinking points. This approach is applicable but
not limited to instances where a crosslinking agent is not used to
prepare the absorbent foam.
If a crosslinking agent is used, it is generally desired that the
crosslinking agent be used in an amount that is beneficially from
about 0.01 weight percent to about 20 weight percent, more
beneficially from about 0.05 weight percent to about 10 weight
percent, and suitably from about 0.1 weight percent to about 5
weight percent, based on the total weight of the polymer and the
crosslinking agent present in an absorbent foam.
In general, a crosslinking catalyst will not be needed, but may be
beneficial, to assist in the crosslinking of the polymer in order
to prepare the absorbent foam of the present invention. For
example, if citric acid is used as the crosslinking agent, sodium
hypophosphite is beneficially used as a crosslinking catalyst. If a
crosslinking catalyst is used, it is generally desired that the
crosslinking catalyst be used in an amount of from about 0.01 to
about 3 weight percent, suitably from about 0.1 to about 1 weight
percent, based on the total weight of the polymer used.
While the principal components of the absorbent foam of the present
invention have been described in the foregoing, such an absorbent
foam is not limited thereto and can include other components not
adversely effecting the desired properties of the absorbent foam.
Exemplary materials which could be used as additional components
would include, without limitation, pigments, antioxidants,
stabilizers, plasticizers, nucleating agents, surfactants, waxes,
flow promoters, solid solvents, particulates, and materials added
to enhance processability of the absorbent foam. If such additional
components are included in an absorbent foam, it is generally
desired that such additional components be used in an amount that
is beneficially less than about 10 weight percent, more
beneficially less than about 5 weight percent, and suitably less
than about 1 weight percent, wherein all weight percents are based
on the total weight amount of the amount of the polymer, any
crosslinking agents, and any other optional components present in
the absorbent foam.
The absorbent foam of the present invention suitably has the
ability to absorb a liquid, herein referred to as the Free Swell
(FS) value. The method by which the Free Swell value is determined
is set forth below in connection with the examples. The Free Swell
values determined as set forth below and reported herein refer to
the amount in grams of an aqueous solution, containing 0.9 weight
percent sodium chloride, a gram of a material can absorb in about 1
hour under a negligible load of about 0.01 pound per square inch
(psi). As a general rule, it is desired that the absorbent foam of
the present invention has a Free Swell value, for a load of about
0.01 psi, of at least about 10, beneficially of at least about 15,
more beneficially of at least about 20, suitably of at least about
25, more suitably of at least about 30, and up to about 200 grams
per gram of absorbent foam.
The absorbent foam of the present invention also suitably has the
ability to absorb a liquid while the absorbent composition is under
an external pressure or load, herein referred to as the Absorbency
Under Load (AUL) value. The ability of a material to absorb a
liquid while the absorbent composition is under an external
pressure or load has been found to often be an important
characteristic of an absorbent material used in a disposable
absorbent product since, while being worn and/or used by a
consumer, the disposable absorbent product is often subjected to an
external pressure or load that may negatively impact on the ability
of the absorbent material being used to effectively absorb any
liquid insulting the disposable absorbent product. The method by
which the Absorbency Under Load is determined is set forth below in
connection with the examples. The Absorbency Under Load values
determined as set forth below and reported herein refer to the
amount in grams of an aqueous solution, containing 0.9 weight
percent sodium chloride, a gram of a material can absorb in about 1
hour under a load of about 0.3 pound per square inch (psi). As a
general rule, it is desired that the absorbent foam of the present
invention has an Absorbency Under Load value, for a load of about
0.3 psi, of at least about 10, beneficially of at least about 15,
more beneficially of at least about 20, suitably of at least about
25, more suitably of at least about 30, and up to about 100 grams
per gram of absorbent foam.
It has been discovered that the conditions under which an absorbent
foam is stored may potentially have an impact on the absorbent
properties of the absorbent foam as it ages. Even relatively mild
conditions, such as ambient conditions, such as about 24.degree. C.
and at least about 30 percent relative humidity, suitably between
about 30 to about 60 percent relative humidity, may result in a
degradation of the absorbent properties of an absorbent foam as it
ages. Typically, storage conditions, such as relatively higher
temperatures and/or relatively higher relative humidities, as
compared to ambient conditions, may result in quicker and/or more
severe degradation of the absorbent properties of an absorbent foam
as it ages.
In one embodiment of the present invention, the absorbent foam of
the present invention will tend to retain its initial Free Swell
and AUL values after aging. Specifically, an absorbent foam of the
present invention may retain greater than about 50 percent, and
suitably greater than about 70 percent, of its initial Free Swell
or AUL values after aging for about 60 days. Typically, the aging
conditions are at ambient conditions, such as at about 24.degree.
C. and at least about 30 percent relative humidity. For example, if
an absorbent foam of the present invention has an initial AUL value
of about 20, that absorbent foam may have an AUL value of at least
about 10, and suitably of about 14, after aging for about 60 days
at about 24.degree. C. and at least about 30 percent relative
humidity. Otherwise similar absorbent foams may tend to not retain
their initial Free Swell or AUL values after aging under similar
conditions.
Suitably, the absorbent foam of the present invention retains
greater than about 50 percent, and more suitably greater than about
70 percent, of their initial Free Swell and AUL values after aging
for about 60 days at about 24.degree. C. and about 100 percent
relative humidity.
As used herein, the term "initial Free Swell" or "initial
Absorbency Under Load" is meant to refer to that Free Swell or AUL
value exhibited by an absorbent foam as measured within about 1 day
after preparation of the absorbent foam when the absorbent foam is
stored at ambient conditions, such as at about 24.degree. C. and
between about 30 to about 60 percent relative humidity.
It is also desirable that the absorbent foam of the present
invention exhibit addition liquid handling properties such as
suitable liquid vertical wicking or liquid intake rate values.
The absorbent foam of the present invention also suitably exhibits
desired softness characteristics, herein quantified by the use of a
Softness value. It is generally desired to have an absorbent foam
that is soft and flexible so that a disposable absorbent product
comprising the absorbent foam will provide a good fit to a wearer
or user of the disposable absorbent product so as to prevent
premature liquid leakage, a certain degree of comfort, and a
reduced packaging volume because a soft material generally provides
a maximum compressibility and folding capacity. The method by which
the Softness value is determined is set forth below in connection
with the examples. The Softness values determined as set forth
below and reported herein refer to the force value that relates to
the stiffness of a material. Using the test method described
herein, the Softness value of a material gives an average of the
stiffness of the material in all directions, and is a measurement
of force exerted on the material at a rate of 50 centimeters per
minute, in a circular bending test. In general, the higher the
force value needed to bend a material, the more stiff the material
is. As a general rule, it is desired that the absorbent foam of the
present invention exhibits a Softness value that is beneficially
less than about 30, more beneficially less than about 25, even more
beneficially less than about 20, suitably less than about 15, more
suitably less than about 10, and even more suitably less than about
5 grams of force per gram per square meter of absorbent foam.
A typical foam will comprise open spaces or cells within the
structure of the foam. In the development of the present invention,
it has been determined that the size of the cells of an absorbent
foam generally affects certain liquid transportation properties,
such as vertical liquid wicking values, but that the size of the
cells of an absorbent foam generally has a minimal affect on the
overall softness or flexibility of the absorbent foam. Such has
been found to be particularly true when the polymer being used to
prepare the absorbent foam is a glassy polymer. Instead, the
softness or flexibility of an absorbent foam has been found to be
generally dependent on the thickness of the cell walls. In general,
the thinner the wall thickness of the cells of an absorbent foam,
the softer and/or more flexible the absorbent foam will be. In
order to achieve the desired absorbency and physical
characteristics of the absorbent foam of the present invention, it
has been found that both the average cell size and the average
thickness of the cell walls of an absorbent foam needs to be
carefully controlled and, preferably, optimized.
In one embodiment of the present invention, it is generally desired
that the average cell size of the cells in an absorbent foam
beneficially be between about 10 microns to about 100 microns and
suitably between about 10 microns to about 50 microns. Such a range
of the average cell size of the cells in an absorbent foam has been
found to generally result in an effective channel system for
distributing liquid within the structure of the absorbent foam. The
method by which the average cell size of the pores in an absorbent
foam is determined is set forth below in connection with the
examples.
In one embodiment of the present invention, it is generally desired
that the average wall thickness of the cells in an absorbent foam
beneficially be between about 0.1 micron to about 30 microns and
suitably between about 0.5 micron to about 10 microns. Such a range
of the wall thickness of the cells in an absorbent foam has been
found to generally result in achieving desired physical properties,
such as softness and/or flexibility, of the absorbent foam. The
method by which the average wall thickness of the pores in an
absorbent foam is determined is set forth below in connection with
the examples.
As used herein, the term "hydrophobic" refers to a material having
a contact angle of water in air of at least 90 degrees. In
contrast, as used herein, the term "hydrophilic" refers to a
material having a contact angle of water in air of less than 90
degrees. For the purposes of this application, contact angle
measurements are determined as set forth in Robert J. Good and
Robert J. Stromberg, Ed., in "Surface and Colloid
Science--Experimental Methods", Vol. II, (Plenum Press, 1979). The
absorbent foams of the present invention are generally hydrophilic
as prepared and therefore generally do not require any subsequent
treatment to make them hydrophilic. This is in contrast to many
absorbent foams known in the art in which the polymeric material of
the foam is not inherently hydrophilic but is rendered hydrophilic
by a suitable treatment, such as by the addition of a
surfactant.
The absorbent foam of the present invention has been found to be
able to be prepared by a relatively simple, safe, and
cost-effective process. In one embodiment, the process generally
comprises forming a solution of a soluble polymer in a solvent,
freezing the solution at a relatively slow cooling rate to a
temperature below the freezing point of the solvent, removing the
solvent from the frozen solution, and optionally treating the
polymer to form a water-swellable, water-insoluble polymeric
absorbent foam.
In another embodiment, the process comprises forming a solution of
monomers in a solvent, polymerizing the monomers to form a solution
gel of a crosslinked polymer in the solvent, freezing the solution
gel at a relatively slow cooling rate to a temperature below the
freezing point of the solvent, and removing the solvent from the
frozen solution gel. Optionally, the solution gel of the
crosslinked polymer could be subjected to additional swelling, by
using additional solvent, before freezing the solution gel.
The absorbent foam of the present invention is also believed to be
capable of being formed by a process generally comprising forming a
solution of a soluble polymer in a solvent, adding a blowing agent
to the solution, initiating the blowing agent, removing the solvent
from the solution, and optionally treating the polymer to form a
water-swellable, water-insoluble polymeric absorbent foam.
As used herein, the term "solvent" is intended to represent a
substance, particularly in a liquid form, that is capable of
dissolving the polymer used herein to form a substantially
uniformly dispersed mixture at the molecular level. In one
embodiment of the present invention, the solvent used to prepare
the absorbent foam needs to be capable of first freezing and then
be capable of undergoing sublimation, wherein the solvent passes
directly from its frozen state to a vapor state. As such, the
solvent used to prepare the absorbent foam should have a freezing
point at which the solvent changes from a liquid to a solid. The
freezing point of water and other solvents is generally well known
in the art. However, as will be recognized by one skilled in the
art, the freezing point of a particular solvent may be affected by
such factors as the particular solvent, polymer and the
crosslinking agents being used as well as the relative
concentrations of the respective components in the solution.
The soluble polymer or the monomers are typically dissolved in a
solvent comprising at least about 30 weight percent water,
beneficially about 50 weight percent water, suitably about 75
weight percent water, and more suitably 100 weight percent water.
When a co-solvent is employed with the water, other suitable
solvents include methanol, ethanol, acetone, isopropyl alcohol,
ethylene glycol, glycerol, and other solvents known in the art.
However, when a water-soluble polymer is used, the use or presence
of such other, non-aqueous solvents may impede the formation of a
homogeneous mixture such that the polymer does not effectively
dissolve into the solvent to form a solution.
In general, a solution of the polymer, the solvent and, optionally,
a crosslinking agent and/or other optional components is prepared,
wherein the polymer may be added to the solution as a polymer or
formed as a polymer in the solution from monomers. In the present
invention, it has been discovered that controlling the
concentration of the polymer in the solution is important to
achieving an absorbent foam that exhibits the desired properties.
In general, if the concentration of the polymer in the solution is
too high, the resultant absorbent foam prepared has been found to
not exhibit the desired properties, particularly softness, due to
the formation of relatively thick cell walls. Without intending to
be bound hereby, it is hypothesized that the use of too great of a
concentration of the polymer in the solution results in a
relatively small volume of space occupied by solvent molecules as
compared to the overall solution volume. In general, if the
concentration of the polymer in the solution is too low, the
resultant absorbent foam prepared has been found to not exhibit the
desired properties, particularly absorbent properties and liquid
distribution capability, due to the formation of cell walls that
are too thin and cells that are too large. Without intending to be
bound hereby, it is hypothesized that the use of too small of a
concentration of the polymer in the solution results in too much
volume of space occupied by the solvent molecules. It has generally
been found that the higher the concentration of the polymer in the
solution, the resulting absorbent foam exhibits a smaller average
cell size and thicker average cell walls as compared to an
absorbent foam prepared from a solution with a lower concentration
of the polymer in the solution.
Thus, it is generally desired that the solution comprises from
about 0.1 to about 30 weight percent, beneficially from about 0.5
to about 20 weight percent, and suitably from about 1 to about 10
weight percent, based on total solution weight, of the polymer. The
solution generally comprises from about 99.99 to about 70 weight
percent, beneficially from about 99.5 to about 80 weight percent,
and suitably from about 99 to about 90 weight percent of the
solvent.
In one embodiment of the present invention, the dissolution of a
soluble polymer into a solvent is believed to result in
entanglement of individual segments of the polymer chains with each
other. Such entanglement results in the polymer chains
interpenetrating one another in the mixture, so that a random,
coil-entangled molecular configuration occurs which is believed to
effectively provide crosslinking points and which assists allowing
for additional crosslinking of the polymer upon further treatment
as, for example, with heat-treatment. To allow for effective
entanglement of individual segments of the polymer with each other,
the solution is suitably allowed to form a stable, homogeneous
solution at equilibrium prior to additional treatment steps to
ensure effective dissolution of the polymer into the solvent. It
will be appreciated that a relatively minor amount of a non-soluble
portion of the polymer may exist that will typically not dissolve
into the solvent. For example, the retained crystalline areas of a
crystalline-crosslinked polymer will typically not dissolve in
water while the non-crystalline areas typically will.
Generally, the order of mixing the polymer or monomers, the solvent
and, optionally, any crosslinking agents is not critical. As such,
either the polymer, the monomers, or the crosslinking agent may be
added to the solvent and then the remaining component subsequently
added, or all components may be added together at the same time.
However, it may be beneficial, when using certain crosslinking
agents, to first add the polymer or monomer and solvent and then to
add the crosslinking agent to the solution.
The solution of the polymer or monomers, solvent and, optionally, a
crosslinking agent can generally be formed at any temperature at
which the polymer or monomers is soluble in the solvent. Generally,
such temperatures will be within the range of from about 10.degree.
C. to about 100.degree. C.
The solution may be acidic (a pH of less than 7), neutral (a pH of
7), or basic (a pH greater than 7). If desired, the solution can be
acidified by the addition of an aqueous solution of an inorganic
acid, such as hydrochloric acid or nitric acid, or an aqueous
solution of an organic acid, such as acetic acid. Similarly, if it
is desired to provide the solution with a basic pH, a base such as
an aqueous solution of sodium hydroxide, potassium hydroxide, or
ammonia can be added to the solution.
The solution will generally have a pH within the range of from
about 2 to about 12, beneficially from about 4 to about 9, more
beneficially from about 4 to about 7.5, and suitably from about 6
to about 7.5. The resulting absorbent foam will generally have the
same pH as the solution.
When the absorbent foam of the present invention is intended for
use in personal care products, such as diapers, training pants, and
feminine care products, it is typically desired that the absorbent
foam have a generally neutral character. For this reason, it is
generally beneficial that the solution be formed with a generally
neutral pH. If the solution is formed with an acidic or basic pH,
the recovered absorbent foam may be acidic or basic (respectively)
but may be neutralized. A recovered absorbent foam which is acidic
may be neutralized, for example, by contacting it with a gaseous
base such as ammonia. A recovered absorbent foam which is basic may
be neutralized, for example, by contacting it with an acidic gas
such as carbon dioxide.
After forming the solution comprising the polymer or monomers,
solvent and, optionally, a crosslinking agent, the solution is
beneficially agitated, stirred, or otherwise blended to effectively
uniformly mix the components such that an essentially homogeneous
solution is formed.
If monomers are being used, the monomers are suitably then treated
to form the desired polymer in the solution.
The solution is then cooled to a temperature that is below the
freezing point of the solvent such that the solvent freezes and
becomes a solid phase in the solution. Since the polymer and,
optionally, a crosslinking agent are essentially homogeneously
dispersed in the solution, it is generally desired that the polymer
and, optionally, the crosslinking agent form an essentially
continuous matrix within the frozen solution when the solvent
freezes and becomes a solid phase. As such, the essentially
continuous matrix of the polymer and, optionally, the crosslinking
agent will become substantially encased by the frozen solvent,
forming an essentially uniform bicontinuous structure. As used
herein, the term "encase" and related terms are intended to mean
that the frozen solvent phase substantially encloses or surrounds
the essentially continuous matrix of the polymer and, optionally,
the crosslinking agent.
As will be recognized by one skilled in the art, the temperature to
which the solution is cooled in order to freeze the solvent will
typically depend on such factors as the solvent, the polymer and
the crosslinking agent being used as well as the relative
concentrations of the respective components in the solution. In
general, it has been found that if the temperature to which to
solution is eventually cooled is too close to the freezing point of
the solvent, the frozen polymer solution may not exhibit sufficient
strength and may deform under further processing steps such as
under vacuum treatment to remove the frozen solvent. In addition,
the freezing point of the solvent may be depressed due to the
effect of the dissolved polymer and/or crosslinking agent. As such,
if the solution is merely cooled to the freezing point of the pure
solvent, then some of the solvent present in the solution may not
be in the frozen state at such a temperature. In general, it has
also been found that if the temperature to which to solution is
eventually cooled is too far below the freezing point of the
solvent, molecules of the solvent in the solution may tend to form
a non-uniform crystalline phase throughout the solution which has
been found to often cause the formation of cracks in the polymer
matrix and thus in the absorbent foam that is being prepared. Such
cracks tend to reduce the mechanical properties of the absorbent
foam, such as tensile strength and softness or flexibility. In
addition, the use of very low temperatures tends to slow down the
rate of subliming the frozen solvent.
In one embodiment where the solvent used to prepare the absorbent
foam is essentially all water or an aqueous solution comprising
mostly water but also another solvent, it is generally desired that
the temperature to which to solution is eventually cooled to be
between about -50.degree. C. and about 0C., beneficially between
about -50.degree. C. and about -5.degree. C., more beneficially
between about -40.degree. C. and about -10.degree. C., and suitably
between about -30.degree. C. and about -10.degree. C.
It has also been found that the rate at which the solution is
cooled from a temperature above the freezing point of the solvent
to a temperature below the freezing point of the solvent is
important to achieving an absorbent foam that exhibits the desired
properties described herein. In a qualitative manner, the cooling
rate used should be not be so fast that visible cracks or visible
non-uniformities begin to form in the freezing solution. As such,
there is generally a critical cooling rate that will exist for a
particular solution in order to achieve a desired absorbent foam of
the present invention. Using a cooling rate that is faster than
such a critical cooling rate will generally result in an
undesirable absorbent foam that exhibits a relatively non-uniform
pore structure and cracked polymer matrix. In contrast, using a
cooling rate that is slower than such a critical cooling rate will
generally result in a desirable absorbent foam that has a
relatively uniform pore structure and the absence of any
significant cracks or deformities in the polymer matrix.
As with the freezing point of a solvent, the critical cooling rate
to be used for a particular solution will typically depend on such
factors as the solvent, the polymer and the crosslinking agent
being used as well as the relative concentrations of the respective
components in the solution. In one embodiment of the present
invention, wherein water is the solvent or an aqueous solution
comprising mostly water but also another solvent and, more
particularly, wherein the polymer is used in a concentration of
between about 0.5 to about 2 weight percent wherein the weight
percent is based on the total weight of the solvent, the critical
cooling rate has been found to be a decrease in temperature between
about 0.4.degree. C. to about 0.5.degree. C. per minute. In such an
embodiment, it is therefore desired that the cooling rate used to
freeze the solvent be less than about 0.4.degree. C. per minute,
beneficially less than about 0.3.degree. C. per minute, and
suitably less than about 0.1.degree. C. per minute.
As will be recognized by one skilled in the art, besides the
approach of using a cooling rate slower than a critical cooling
rate to achieve an essentially uniform cell structure in the
absorbent foam, other methods can also be applied. Such other
methods include, but are not limited to, the inclusion of tiny air
bubbles or the use of a nucleating agent. Without intending to be
bound hereby, it is hypothesized that the use of a nucleating agent
will increase the number of nuclei to ensure an essentially uniform
crystallization of solvent molecules during the cooling process.
Use of a nucleating agent generally increases the critical cooling
rate.
After the solution has been cooled such that the solvent freezes
and becomes a solid phase in the solution and the solution has
beneficially reached a relatively stable temperature, the frozen
solvent is then substantially removed from the solution. In the
present invention, the use of a suitable vacuum to sublime the
frozen solvent has been found to generally result in a desired
absorbent foam. As will be appreciated by one skilled in the art,
the vacuum to be used for a particular frozen solution will
typically depend on such factors as the solvent, the polymer and
the crosslinking agent being used, the relative concentrations of
the respective components in the solution, and the temperature of
the frozen solution. Desirable vacuum conditions are beneficially
less than about 500 millitorrs, more beneficially less than about
300 millitorrs, suitably less than about 200 millitorrs, and more
suitably less than about 100 millitorrs. In general, the higher the
vacuum, the faster the rate of sublimation of the frozen
solvent.
As used herein, the sublimation, by use of a vacuum, of the frozen
solvent from the frozen solution is meant to represent that
substantially all of the solvent is removed from the frozen
solution prior to, if needed, any additional treatment steps. It
will be appreciated, however, that even after removal of
substantially all of the solvent, a small amount of solvent may
remain entrapped within the structure of the remaining polymeric
matrix. The amount of solvent remaining entrapped within the
structure of the polymeric matrix will typically depend on the
method and conditions under which the frozen solvent is sublimed.
Generally, less than about 20 weight percent, beneficially less
than about 15 weight percent, and suitably less than about 10
weight percent, of the original amount of solvent in the solution
will remain entrapped within the remaining polymeric matrix of the
absorbent foam.
After the frozen solvent has been substantially sublimed from the
frozen solution, the polymer and, optionally, any crosslinking
agent will remain, with the polymer generally forming a polymeric
matrix comprising generally interconnected cells to achieve a foam
structure. The polymeric matrix forms the walls of the cells with
the open cells having been created by the sublimation of the frozen
solvent. As discussed hereinbefore, it is generally desired that
the resultant foam structure exhibit a desired average pore size
and a desired average thickness of the cell walls.
The recovered foam structure may already exhibit the desired
absorbent and physical properties such that the recovered foam
structure is an absorbent foam of the present invention and does
not require any further treatment steps. As will be appreciated by
one skilled in the art, this will generally depend on the
particular polymer and, if used, the particular crosslinking agent
used in the preparation of the foam. Methods of preparation wherein
the recovered foam structure may already exhibit the desired
absorbent and physical properties include wherein monomers were
polymerized in the solution to form a crosslinked and thus
insoluble polymer gel solution; a crosslinking agent that is
capable of reacting with the polymer at a relatively low
temperature, such as at about room temperature or less, is used;
and the polymer used, such as polyvinyl alcohol or chitosan, is
capable of forming a highly ordered structure during the freezing
and solidification process.
If the recovered foam structure does not yet exhibit the desired
absorbent and physical properties, it may be necessary to treat the
recovered polymeric foam structure with an additional process step.
For example, if the crosslinking agent used is a latent
crosslinking agent, such a crosslinking agent may not yet have
reacted with the polymer because the proper crosslinking condition
has not yet been provided to the polymer and crosslinking agent
mixture. As such, an effective crosslinking condition may still
need to be provided in order to crosslink the polymer to achieve a
water-insoluble, water-swellable polymer. Suitable post treatment
conditions include using heat treatment, exposure to ultraviolet
light, exposure to microwaves, exposure to an electron beam, steam
or high humidity treatment, high pressure treatment, or treatment
with an organic solvent.
In general, if heat-treatment is necessary, any combination of
temperature and time which is effective in achieving a desired
degree of crosslinking, without undesirable damage to the polymer,
so that the polymer and the absorbent foam exhibit the desired
properties described herein, is suitable for use in the present
invention. As a general rule, when a crosslinking agent is used,
the polymer will be heat-treated at a temperature between about
50.degree. C. to about 250.degree. C., beneficially from about
80.degree. C. to about 250.degree. C., more beneficially from about
100.degree. C. to about 200.degree. C., and suitably from about
100.degree. C. to about 160.degree. C. The higher the temperature
employed, the shorter the period of time generally necessary to
achieve the desired degree of crosslinking. It has been found that
if very high temperatures are used with an effective length of
time, such as a temperature between about 100.degree. C. and about
250.degree. C. for a length of time between about 50 seconds and
about 500 minutes, effective Free Swell and Absorbency Under Load
values may be achieved for certain polymers, such as carboxyalkyl
polysaccharide without the use of a crosslinking agent.
Generally, the heat-treating process will extend over a time period
within the range of from about 1 minute to about 600 minutes,
beneficially from about 2 minutes to about 200 minutes, and
suitably from about 5 minutes to about 100 minutes.
If used, a heat-treating process, or any other acceptable
post-recovery treatment process, generally causes the polymer to
crosslink or additionally crosslink and become generally water
swellable and water insoluble. Without intending to be bound
hereby, it is believed that the post-recovery treatment processes
cause the polymer to undergo a degree of crosslinking, not related
to the presence of a crosslinking agent, through the formation of
crosslinks between either the functional groups from the polymer
and the external crosslinking agent or between the functional
groups on the polymer when it contains more than one type of
functional groups. One example of a self-crosslinkable polymer is
carboxymethyl cellulose, which contains both carboxylic acid groups
and hydroxyl groups and is able to form ester linkages. This
self-crosslinking may be in addition to any crosslinking caused by
the presence of a crosslinking agent. Further, when the
crosslinking agent is a diamine or polyamine, it is believed that
crosslinking occurs through amidation of any carboxyl groups on the
polymer through the formation of an ammonia salt. Esterification,
through a self-crosslinking process, is believed to occur primarily
under a weakly acidic, neutral, or slightly basic condition.
Esterification, through a self-crosslinking process, is not
believed to proceed to a significant degree under relatively basic
conditions. Crosslinking due to the crosslinking agent may occur
under both acidic and basic conditions. Thus, the presence of the
crosslinking agent allows for crosslinking to occur over a broad pH
range.
There is generally an optimum degree or amount of crosslinking of a
particular polymer that optimizes the absorbency properties of the
particular crosslinked polymer. If too little crosslinking occurs,
the polymer may possess relatively low absorbency properties, such
as Absorbency Under Load values, due to a lack of gel strength. If
too much crosslinking occurs, the polymer may similarly possess
relatively low absorbency properties, such as Free Swell values,
due to the inability of the polymer to absorb liquid.
Those skilled in the art will recognize that the presence of
crosslinks formed by esterification or amidation, ionic bonding, or
other types of linkages can be detected through various analytical
techniques. For example, infrared spectroscopy and nuclear magnetic
resonance can be used to verify the presence of ester and amide
crosslinks.
The absorbent foams of the present invention are suited for use in
disposable products including disposable absorbent products such as
diapers, adult incontinent products, and bed pads; in catamenial
devices such as sanitary napkins, and tampons; and other absorbent
products such as wipes, bibs, wound dressings, and surgical capes
or drapes. Accordingly, in another aspect, the present invention
relates to a disposable absorbent product comprising the absorbent
foams of the present invention.
In one embodiment of the present invention, a disposable absorbent
product is provided, which disposable absorbent product comprises a
liquid-permeable topsheet, a backsheet attached to the
liquid-permeable topsheet, and an absorbent structure positioned
between the liquid-permeable topsheet and the backsheet, wherein
the absorbent structure comprises an absorbent foam of the present
invention.
Absorbent products and structures according to all aspects of the
present invention are generally subjected, during use, to multiple
insults of a body liquid. Accordingly, the absorbent products and
structures are desirably capable of absorbing multiple insults of
body liquids in quantities to which the absorbent products and
structures will be exposed during use. The insults are generally
separated from one another by a period of time.
Test Methods
Free Swell
The Free Swell Capacity (FS) is a test which measures the amount in
grams of an aqueous solution, containing 0.9 weight percent sodium
chloride, a gram of a material can absorb in 1 hour under a
negligible applied load or restraining force, such as of about 0.01
pound per square inch.
Referring to FIG. 1, the apparatus and method for determining the
Free Swell and the Absorbency Under Load will be described. Shown
is a perspective view of the apparatus in position during a test.
Shown is a laboratory jack 1 having an adjustable knob 2 for
raising and lowering the platform 3. A laboratory stand 4 supports
a spring 5 connected to a modified thickness meter probe 6, which
passes through the housing 7 of the meter, which is rigidly
supported by the laboratory stand. A plastic sample cup 8, which
contains the absorbent foam material sample to be tested, has a
liquid-permeable bottom and rests within a Petri dish 9 which
contains the saline solution to be absorbed. For the determination
of Absorbency Under Load values only, a weight 10 rests on top of a
spacer disc (not visible) resting on top of the absorbent foam
material sample (not visible).
The sample cup consists of a plastic cylinder having a 1 inch
inside diameter and an outside diameter of 1.25 inches. The bottom
of the sample cup is formed by adhering a 100 mesh metal screen
having 150 micron openings to the end of the cylinder by heating
the screen above the melting point of the plastic and pressing the
plastic cylinder against the hot screen to melt the plastic and
bond the screen to the plastic cylinder.
The modified thickness meter used to measure the expansion of the
sample while absorbing the saline solution is a Mitutoyo Digimatic
Indicator, IDC Series 543, Model 543-180, having a range of 0-0.5
inch and an accuracy of 0.00005 inch (Mitutoyo Corporation, 31-19,
Shiba 5-chome, Minato-ku, Tokyo 108, Japan). As supplied from
Mitutoyo Corporation, the thickness meter contains a spring
attached to the probe within the meter housing. This spring is
removed to provide a free-falling probe which has a downward force
of about 27 grams. In addition, the cap over the top of the probe,
located on the top of the meter housing, is also removed to enable
attachment of the probe to the suspension spring 5 (available from
McMaster-Carr Supply Co., Chicago, Ill., Item No. 9640K41), which
serves to counter or reduce the downward force of the probe to
about 1 gram+0.5 gram. A wire hook can be glued to the top of the
probe for attachment to the suspension spring. The bottom tip of
the probe is also provided with an extension needle (Mitutoyo
Corporation, Part No. 131279) to enable the probe to be inserted
into the sample cup.
To carry out the test, an absorbent foam material sample was cut
into circular discs with a diameter of about one inch. A total of
about 0.160 gram of the absorbent foam material sample, typically
about 3 to 4 circular disc layers, is placed into the sample cup.
The sample is then covered with a plastic spacer disc, weighing 4.4
grams and having a diameter of about 0.995 inch, which serves to
protect the sample from being disturbed during the test and also to
uniformly apply a load on the entire sample. The sample cup, with
material sample and spacer disc, is then weighed to obtain its dry
weight. The sample cup is placed in the Petri dish on the platform
and the laboratory jack raised up until the top side of the plastic
spacer disc contacts the tip of the probe. The meter is zeroed. A
sufficient amount of saline solution is added to the Petri dish
(50-100 milliliters) to begin the test. The distance the plastic
spacer disc is raised by the expanding sample as it absorbs the
saline solution is measured by the probe. This distance, multiplied
by the cross-sectional area inside the sample cup, is a measure of
the expansion volume of the sample due to absorption. Factoring in
the density of the saline solution and the weight of the sample,
the amount of saline solution absorbed is readily calculated. The
weight of saline solution absorbed after about 1 hour is the Free
Swell value expressed as grams saline solution absorbed per gram of
absorbent foam sample. If desired, the readings of the modified
thickness meter can be continuously inputted to a computer
(Mitutoyo Digimatic Miniprocessor DP-2 DX) to make the calculations
and provide Free Swell readings. As a cross-check, the Free Swell
can also be determined by determining the weight difference between
the sample cup before and after the test, the weight difference
being the amount of solution absorbed by the sample.
Absorbency Under Load
The Absorbency Under Load (AUL) is a test which measures the amount
in grams of an aqueous solution, containing 0.9 weight percent
sodium chloride, a gram of a material can absorb in 1 hour under an
applied load or restraining force of about 0.3 pound per square
inch. The procedure for measuring the Absorbency Under Load value
of an absorbent composition is essentially identical to the
procedure for measuring the Free Swell values, except that a 100
gram weight is placed on top of the plastic spacer disc, thereby
applying a load of about 0.3 pound per square inch onto the
absorbent foam as it absorbs the saline solution.
Softness
The Softness value of a material is determined by a test which is
modeled after the ASTM D4032-82 Circular Bend Procedure. This
modified test is used for the purposes of the present invention and
is, hereinafter, simply referred to as the "Circular Bend
Procedure". The Circular Bend Procedure is a simultaneous
multi-directional deformation of a material in which one face of a
material becomes concave and the other face becomes convex. The
Circular Bend Procedure gives a force value which relates to the
stiffness of the material, simultaneously averaging stiffness in
all directions, and is herein as being inversely related to the
softness of the material.
The apparatus necessary for the Circular Bend Procedure is a
modified Circular Bend Stiffness Tester, having the following
parts: A smooth-polished steel plate platform which is 102.0
millimeters (length) by 102.0 millimeters (width) by 6.35
millimeters (depth) having a 18.75 millimeter diameter orifice. The
lap edge of the orifice should be at a 45 degree angle to a depth
of 4.75 millimeters. A plunger having the following dimensions is
used: overall length of 72.2 millimeters, a diameter of 6.25
millimeters, a ball nose having a radius of 2.97 millimeters and a
needle-point extending 0.88 millimeters from the ball nose with a
0.33 millimeter base diameter and a point having a radius of less
than 0.5 millimeters. The plunger is mounted concentrically with
the orifice having equal clearance on all sides. The needle-point
is used merely to prevent lateral movement of a sample during
testing. The bottom of the plunger should be set well above the top
of the orifice plate. From this position, the downward stroke of
the ball nose is to the exact bottom of the plate orifice.
An inverted compression load cell having a load range of from about
0.0 to about 2000.0 grams was used as a force measurement gauge.
The compression tester used was an Instron Model No. 1122 inverted
compression load cell, available from Instron Engineering
Corporation of Canton, Mass.
After calibrating the load cell, the gage length for displacement
of the plunger was set to 25.4 mm. To carry out the test, an
absorbent foam sample was cut into a 38.1.times.38.1 mm square
specimen using a die cutter. The sample was placed onto the test
platform and the plunger was lowered down on the specimen for a
25.4 mm gage length at a crosshead speed of 500 mm/min. During the
movement of the plunger, the absorbent foam sample is deflected
downward into the 18.75 mm hole by the plunger and the force
exerted by the compression tester to deflect the foam sample during
the 25.4 mm gage length displacement of the plunger is measured by
the load cell and recorded. The force measured by the load cell
divided by the basis weight of the specimen is reported in units of
grams force/grams per square meter of specimen (g/gsm). This value
is used as the Softness value to obtain a quantitative measure of
the softness of the specimen. The higher the Softness value (in
g/gsm), the more stiff and, thus, the less soft, the specimen.
Cell Pore Size and Cell Wall Thickness Measurements
A foam sample was cut by a sharp razor. The cut foams were attached
to metal stubs using copper tape and imaged in an environmental
scanning electron microscope using 12 kV beam voltage. The
instrument used was an environmental scanning electron microscope,
model E-2020 from Electroscan Corporation of Wilmington, Mass. The
sample chamber pressure was about 1.2 Torr. The environmental
backscatter electron detector was used to collect images, having
the advantage of being able to discern any variations in
composition. Magnification varied depending on the scale of the
subject sample, with a 150 magnification used for a general survey
of the sample and a 2500 magnification used to measure cell wall
thickness and cell size. Cell wall thickness and cell size
measurements were taken directly on the environmental scanning
electron microscope. It was not possible to apply automated image
analysis routines to these complex structures for cell wall
thickness measurement. Manual measurement is required. The cell
wall thickness and cell size of each sample are averaged from at
least measurements.
EXAMPLES
For use in the following examples, the following polymer materials
were obtained.
Polymer 1: A carboxymethylcellulose having a weight average
molecular weight greater than 1,000,000 and a degree of
substitution of carboxymethyl groups on the anhydroglucose unit of
the cellulosic material of about 0.7 was obtained from Aqualon of
Wilmington, Del., a subsidiary of Hercules Inc., under the
designation B313 carboxymethylcellulose. Carboxymethylcellulose is
an anionic polymer.
Polymer 2: A sodium polyacrylate polymer having a weight average
molecular weight of about 4,000,000 and degree of neutralization of
about 70 percent was obtained from Polysciences of Warrington, Pa.,
under the catalog number of 06501. Sodium polyacrylate polymer is
an anionic polymer.
Polymer 3: A sodium polyacrylate polymer having a weight average
molecular weight of about 240,000 and degree of neutralization of
about 70 percent was obtained from Polysciences of Warrington, Pa.,
under the catalog number of 18613. Sodium polyacrylate polymer is
an anionic polymer.
Polymer 4: A sodium polyacrylate polymer having a weight average
molecular weight of about 60,000 and degree of neutralization of
about 70 percent was obtained from Polysciences of Warrington, Pa.,
under the catalog number of 18611. Sodium polyacrylate polymer is
an anionic polymer.
Polymer 5: A chitosan acetate having a weight average molecular
weight of about 11,000,000 and degree of acetylation of about 80
percent was obtained from Vanson Company of Seattle, Wash., under
the designation VNS-608 chitosan. Chitosan acetate is a cationic
polymer.
Polymer 6: A polyethyleneoxide having a weight average molecular
weight of about 4,000,000 was obtained from Union Carbide
Corporation of Danbury, Conn., under the designation WSR-301
polyethyleneoxide. Polyethyleneoxide is a nonionic polymer.
Example 1
Weight amounts of the various polymer samples were dissolved in
separate batches of about 2000 grams of distilled water at a
temperature of about 23.degree. C. For the carboxymethylcellulose
(Polymer 1) and polyethylene oxide (Polymer 6) solutions, about 0.2
gram of citric acid was also added to the solutions as a
crosslinking agent. For the sodium polyacrylate solutions (Polymers
2-4) solutions, about 0.75 gram of an aqueous solution comprising
about 40 weight percent of ammonium zirconium carbonate was also
added to the solutions as a crosslinking agent. The various
solutions were blended for about 2 to 3 hours to ensure thorough
mixing of the components. About 500 grams of each prepared solution
was placed into separate stainless steel pans, wherein the pans had
dimensions of 10 inches (width) by 20 inches (length) by 1 inch
(depth). The pans, containing the respective solutions, were then
placed in a freeze dryer, available from The VirTis, Inc., of
Gardiner, N.Y., under the designation VirTis Genesis model 25EL
freeze dryer. The various solutions in the pans were then cooled
down to about -15.degree. C. at various cooling rates in order to
freeze the water in the solutions. The various solutions in the
pans were maintained at about -15.degree. C. for about an hour to
ensure substantially complete freezing of the water. The frozen
solutions were left in the freeze dryer and then subjected to a
vacuum of about 105 millitorrs, provided by a vacuum pump which had
a condenser set to a temperature of about -60.degree. C. to about
-70.degree. C., for about 15 hours. The resultant foam structures
were then treated at various temperatures for various periods of
time in order to assist in the crosslinking of the polymers. The
final foam structures were then evaluated for Free Swell,
Absorbency Under Load, and Softness values. The various process
conditions and results of the evaluations for the various samples
are summarized in Table 1. The foam sample prepared using Polymer 4
(Sample 7) was water soluble and therefore did not exhibit any
measurable Free Swell and Absorbency Under Load values.
A comparative foam sample (Sample 10) was also prepared, as
follows. About 250 grams of aqueous acrylic acid solution
containing 50 percent by weight of acrylic acid was neutralized
using 1 N sodium hydroxide solution to form sodium acrylate
solution with 75 percent degree of neutralization. The
neutralization was carried out slowly using an ice bath taking care
to maintain the solution temperature around 5.degree. C. to avoid
any polymerization. About 200 ml of this solution was transferred
to a 2 liter reaction vessel fitted with a heating jacket and a
high shear mixer (Ultra-Turrax T25 mixer from Janke & Kunkel
GmbH of Staufen, Germany). To the solution in the reaction vessel
was added 0.5 grams of N,N'-methylenebisacrylamide, about 1.3 grams
of 2,2'-azobis-(2-amidopropane) hydrochloride from Monomer-Polymer
& Dajac Laboratories, Inc. of Feasterville, Pa., and about 20
grams of polyethylene glycol of 600 weight average molecular weight
from Union Carbide Company maintaining the mixture at 22.degree. C.
About 3.5 grams of sorbitan monolaurate and about 6.5 grams of
ethoxylated sorbitan monolaurate were mixed in about 60 grams of
1,1,2-trichlorotrifluoroethane and this mixture was then added to
the solution in the reaction vessel. The high shear mixer was
started and the mixture was mixed at a speed of about 8000 rpm for
about 10 minutes after which the mixer was removed from the
reaction vessel and the temperature increased to about 60.degree.
C. and maintained for about 1 hour to form the foam, followed by
increasing and maintaining the temperature at about 80.degree. C.
for about 30 minutes and finally increasing and maintaining the
temperature at about 120.degree. C. for about 30 minutes. The
reactor was then cooled to about 22.degree. C. A mixture consisting
of about 5 grams of glycerol and about 25 grams of isopropyl
alcohol was added to the foam in the reactor and the temperature
was increased to about 180.degree. C. and maintained for about 1
hour. The reactor was then cooled to ambient temperature, the foam
removed from the reactor and placed in a chamber at about 80
percent relative humidity for about 6 hours to obtain the final
foam material. This foam sample was then evaluated for Free Swell,
Absorbency Under Load, and Softness values, with the results of
such evaluations also summarized in Table 1.
Example 2
Multiple, substantially similar foam samples were prepared as
follows. About 10 grams of Polymer 1 (carboxymethylcellulose) was
dissolved in about 2000 grams of distilled water at a temperature
of about 23.degree. C. About 0.2 gram of citric acid was also added
to the solution as a crosslinking agent. The solution was blended
for about 2 to 3 hours to ensure thorough mixing of the components.
About 500 grams of the solution was placed into a stainless steel
pan, wherein the pan had dimensions of 10 inches (width) by 20
inches (length) by 1 inch (depth). The pan, containing the
solution, was then placed in freeze dryer, available from The
VirTis, Inc. of Gardiner, N.Y., under the designation VirTis
Genesis model 25EL freeze
TABLE 1
__________________________________________________________________________
Polymer Heat Concentration Treatment Absorbency Softness Polymer
(Weight Cooling Conditions Free Swell Under Load Value Sample No.
Type Percent) Rate (Temperature/Time) Value (g/g) Value (g/g)
(g/gsm)
__________________________________________________________________________
Sample 1 Polymer 1 0.5 0.03.degree. C./min 130.degree. C./10 min
26.1 18.3 2.90 Sample 2 Polymer 1 0.5 0.2.degree. C./min
130.degree. C./10 min 21.8 -- 6.53 Sample 3 Polymer 1 0.5
0.4.degree. C./min 130.degree. C./10 min 22.1 -- 20.58 *Sample 4
Polymer 1 4.0 0.4.degree. C./min 130.degree. C./10 min 18.7 --
45.93 Sample 5 Polymer 2 0.5 0.03.degree. C./min 200.degree. C./40
min 35.0 19.5 1.36 Sample 6 Polymer 3 0.5 0.03.degree. C./min
200.degree. C./5 hours 42.8 2.5 1.09 *Sample 7 Polymer 4 0.5
0.03.degree. C./min 200.degree. C./72 hours 0 0 1.54 Sample 8
Polymer 5 0.5 0.03.degree. C./min 100.degree. C./10 min 22.5 14.3
3.26 Sample 9 Polymer 6 0.5 0.03.degree. C./min 60.degree. C./10
hours 15.3 4.1 0.46 *Sample 10 -- -- -- 15 10.5 >100
__________________________________________________________________________
*Not an example of the present invention.
dryer. The solution in the pan was then cooled down to about
-15.degree. C. at a cooling rate of about 0.04.degree. C./minute in
order to freeze the water in the solution. The solution in the pan
were maintained at about -15.degree. C. for about an hour to ensure
substantially complete freezing of the water. The frozen solutions
were left in the freeze dryer and then subjected to a vacuum of
about 105 millitorrs, provided by a vacuum pump which had a
condenser set to a temperature of about -60.degree. C. to about
-70.degree. C., for about 15 hours.
The resultant foam structures were then treated at various
temperatures for various periods of time in order to assist in the
crosslinking of the polymers. The final foam structures were then
evaluated for Free Swell, Absorbency Under Load, and Softness
values. The various process conditions and results of the
evaluations for the various samples are summarized in Table 2.
TABLE 2 ______________________________________ Heat Treatment
Conditions Absorbency Softness (Temperature/ Free Swell Under Load
Value Sample No. Time) Value (g/g) Value (g/g) (g/gsm)
______________________________________ *Sample 11 None 0 0 20.58
Sample 12 150.degree. C./5 min 27.9 -- -- Sample 13 150.degree.
C./10 min 25.1 14.3 -- Sample 14 150.degree. C./20 min 15.7 11.2 --
Sample 15 150.degree. C./30 min 16.6 11.3 20.58 *Sample 16 None 0 0
-- Sample 17 130.degree. C./5 min 60.1 29.9 20.58 Sample 18
130.degree. C./10 min 26.1 18.3 -- Sample 19 130.degree. C./15 min
21.8 16.2 -- Sample 20 130.degree. C./20 min 18.2 14.6 -- Sample 21
130.degree. C./25 min 17.4 13.9 20.58
______________________________________ *Not an example of the
present invention.
Those skilled in the art will recognize that the present invention
is capable of many modifications and variations without departing
from the scope thereof. Accordingly, the detailed description and
examples set forth above are meant to be illustrative only and are
not intended to limit, in any manner, the scope of the invention as
set forth in the appended claims.
* * * * *